In the refining of lubricant base stocks, a series of generally subtractive processes is employed to remove undesirable components from the process feedstock. The most important of these processes include atmospheric and vacuum distillation, deasphalting, solvent extraction, and dewaxing. These processes are basically physical separation processes in the sense that if all the separated fractions were recombined the crude oil would be reconstituted.
Refineries do not manufacture a single lube base stock but rather process several distillate fractions and a vacuum residuum fraction. Atmospheric distillation of crude oil produces several fractions, depending upon the type of crude available and the operations of the refinery. Two fractions are generally considered middle distillates. These are straight run kerosine, with a boiling range of 345.degree.-510.degree. F. and light gas oil, with a boiling range of approximately 510.degree.-700.degree. F. Products derived from these fractions include kerosine, jet fuel, diesel fuel, home heating oil and marine fuel. The lighter middle distillates are also used as specialty lubricants, such as mineral seal oils, spindle oils, honing oils and cold-rolling oils for aluminum, steel, copper and the like.
Generally, at least three vacuum distillate fractions differing in boiling range and the residuum may be refined. These four fractions have acquired various names in the refining art, the most-volatile vacuum distillate fraction often being referred to as the "light neutral" fraction or oil. The other vacuum distillates are known as "intermediate neutral" and "heavy neutral" oils. The vacuum residuum, after deasphalting, solvent extraction and dewaxing, is commonly referred to as "bright stock." Thus, the manufacture of lubricant base stocks involves a process for producing a slate of base stocks. Additionally, each subtractive step produces a byproduct which may be processed further or sold to an industry which has developed a use for the byproduct.
As mentioned, conventional processing of crude petroleum oil to recover fractions suitable for upgrading in various refinery processing operations employs multi-stage distillation. Crude oil is first distilled or fractionated in an atmospheric distillation tower, with residual material from the bottom of the distillation tower being further separated in a vacuum distillation tower. In this combination operation, gas and gasoline generally are recovered as overhead products of the atmospheric distillation tower, heavy naphtha, kerosene and gas oils are taken off as distillate side streams and the residual material is recovered from the bottom of the tower as reduced crude. Steam may be introduced to the bottom of the tower and various side strippers used to remove light material from withdrawn heavier liquid products. The residual bottoms fraction or reduced crude is usually charged to a vacuum distillation tower. The vacuum distillation step in lube refining provides one or more raw stocks within the boiling range of about 550.degree. F. to 1050.degree. F., as well as the vacuum residuum byproduct. Often the vacuum charge is heated by a furnace means in order to vaporize a portion of the charge. The preheated charge normally enters a lower portion of the vacuum tower and the vapors therefrom rise through the tower where they are cooled in selected stages producing successively lighter liquids which are separately withdrawn as the sidestream raw stock products.
Following vacuum distillation, each raw stock is extracted with a solvent, e.g. furfural, phenol or chlorex, which is selective for aromatic hydrocarbons, removing undersirable components. The vacuum residuum usually requires an additional step, typically propane deasphalting, to remove asphaltic material prior to solvent extraction. The products produced for further processing into base stocks are known as raffinates. The raffinate from solvent refining is thereafter dewaxed by admixing with a solvent such as a blend of methyl ethyl ketone and toluene, for example and then processed into finished base stocks.
The solvent extraction step separates hydrocarbon mixtures into two phases; the previously described raffinate phase which contains substances of relatively high hydrogen to carbon ratio, often called paraffinic type materials, and an extract phase which contains substances of relatively low hydrogen to carbon ratio often called aromatic type materials. Solvent extraction is possible because different liquid compounds have different solution affinities for each other and some combinations are completely miscible while other combinations are almost immiscible. The ability to distinguish between high hydrogen to carbon paraffinic type and low hydrogen to carbon aromatic type materials is termed selectivity. The more finely this distinguishing can be done, the higher the selectivity of the solvent. Furfural is typical of a suitable solvent extraction agent. Its miscibility characteristics and physical properties permit use with both highly aromatic and highly paraffinic oils of wide boiling range. Diesel fuels and light and heavy lubricating stocks are often refined with furfural.
Some chemical compounds have been shown to cause skin cancer in humans and laboratory animals following skin contact. In recent years, concerns have arisen regarding the potential hazards associated with the use of various lubricating oils, middle distillates and petroleum products, in general. Previous studies of the higher boiling fractions recovered from vacuum distillation and processed to formulate engine oils and other lubricants have established a fairly consistent pattern of the types of petroleum-derived materials which cause tumors in laboratory animals. Extensively treated oils, such as those treated by solvent refining and severe hydroprocessing are known to have only trace amounts of polycyclic aromatic hydrocarbons (PAH). As such, these oils are generally not tumorigenic; while oils having high PAH levels, especially those compounds of four or more rings, are.
The middle distillates used as specialty oils possess the potential for significant human exposure due to the nature of their industrial applications. Because such straight-run middle distillates boil below 700.degree. F. and typically contain only small levels of PAH compounds, they would not be expected to cause tumors in tests conducted in laboratory animals. However, experiments using laboratory animals have shown this to not be the case.
An indication of tumorigenicity is the time until tumors are developed and observed. Materials that induce skin tumors in a large proportion of laboratory mice typically have a relatively short latency period, and conversely, when the latency period is long few mice develop tumors. It has been found that exposure to middle distillate-based lubricants, however, does not follow this trend. Although several investigations have either not used laboratory animals or long-term topical administration and, as such, have failed to identify the dermal carcinogenicity of kerosine- and middle distillate-based lubricants, one study has shown that a large proportion of mice developed tumors when exposed to such materials over an extended period of time, finding a mean latency period of 79 weeks. (See: G. R. Blackburn, R. A. Deitch, C. A. Schreiner, M. A. Mehlman and C. R. Mackerer, "Predicting Carcinogenicity of Petroleum Fractions Using a Modified Salmonella Mutagenicity Assay", Cell Biology and Toxicology, 2:63-84, 1986). Since industry workers are frequently exposed to middle distillate-based lubricants both continuously and over long periods of time, the finding that such materials can cause tumors is of much concern.
Therefore, what is needed is a non-carcinogenic light industrial lubricant and a process for the production of such a lubricant.